This invention relates to coatings systems, such as for cookware, which minimize sticking. More particularly, it relates to multilayer coating systems which provide concentration gradients within the coating.
U.S. Pat. No. 4,087,394 -- Concannon (1987) discloses aqueous concentration gradient coatings of fluoropolymer which is 20-80% of a homopolymer of polytetrafluoroethylene (PTFE) or a copolymer of fluorinated ethylene-propylene (FEP) made of 5-100% tetrafluoroethylene (TFE) with 95-0% hexafluoropropylene (HFP), with 80-20% of a film forming polymer which can be polyamide-imide (PAI). The coating is applied by spraying onto aluminum sheet, or a variety of substrates. Other application techniques are mentioned. Nothing is said about substrate preparation. Although PTFE and FEP are treated as a continuum, there are no suggestions to use a blend such as 50% PTFE, 50% FEP.
Representative of nonaqueous perfluoropolymer concentration gradient patents are U.S. Pat. No. 3,661,831 (1972) and 4,143,204 (1979) on FEP (preferably FEP has 3-20% HFP, balance TFE) with various film formers including PAI. At the end of each specification, example 24 teaches using such a coating as a primer under an ordinary PTFE topcoat on a frypan. U.K. No. 1,230,339 (1971) claims articles coated with nonaqueous PTFE-PAI with a concentration gradient. This and equivalent Canadian No. 887,122 -- Fang (1971) provide a single coating of PAI and PTFE on a metal substrate with a concentration gradient from mostly PAI at the substrate to mostly PTFE at the air interface. This is applied as a single coat without any special primer on ordinary steel or sand-blasted aluminum.
A mixture of FEP and PTFE in an organic dispersion is in U.S. Pat. No. 3,904,575 (1975) - Satokawa. This says that lower molecular weight PTFE permits more FEP to be included in a stable dispersion. The use of PAI and other film formers is also disclosed. The examples teach sand blasting an aluminum substrate before applying the coating.
Also, U.K. No. 1,454,255 -- Berghmans and Seymus (1976) discloses aqueous dispersion coatings of mixtures of PTFE and FEP with SiO.sub.2 and aluminum phosphate applied preferably to grit-blasted or frit-coated aluminum, but also specifically to untreated aluminum.
U.S. Pat. No. 4,287,112 -- Berghmans (1981) discloses PPS with PTFE, FEP and/or a copolymer of tetrafluoroethylene and hydrocarbon ether monomer which have been completely substituted with fluorine atoms as described in U.S. Pat. No. 4,292,859 (1981) and 4,351,883 (1982) -- both Concannon, known as PFA, along with Al flake and TiO.sub.2 pigment, in both aqueous and non-aqueous coatings. The preferred PTFE is irradiated micropowder. PTFE micropowder may be made according to the teachings of U.S. Pat. No. 3,116,226 -- Bowers (1963), U.S. Pat. No. 4,029,890 -- Brown et al (1977) or U.S. Pat. No. 4,200,551 -- Derbyshire (1980).
Coatings systems including intermediate and top coats are described in various patents including:
U.S. Pat. No. 4,049,863 -- Vassiliou (1977); PA1 U.S. Pat. No. 4,118,537 -- Vary and Vassiliou (1978); PA1 U.S. Pat. No. 4,123,401 -- Berghmans and Vary (1978); PA1 U.S. Pat. No. 4,252,859 -- Concannon and Vary (1981); PA1 U.S. Pat. No. 4,351,882 -- Concannon (1982).